My career goal is to become an established independent physician-scientist in the field of traumatic brain injury. After finishing my emergency medicine residency in 2004, I have pursued additional training through a research fellowship at the University of Pennsylvania that includes enrollment in the neuroscience doctoral program. I chose Penn because it truly embodies the spirit of multidisciplinary collaborative research and training. My five-year career development plan is complete with hypothesis-driven experiments, structured mentorship, and organized didactics. I propose a series of innovative experiments to elucidate the fundamental mechanism of axonal degeneration after brain trauma. My overarching hypothesis is that calpain-mediated proteolysis of microtubule components within minutes of axonal stretch injury impairs microtubule re-polymerization leading to focally disrupted axonal transport and subsequent axonal degeneration. By the end of five years, I will complete the experiments outlined below to test our hypothesis, earn my doctoral degree in neuroscience, and submit my first NIH R01 application. Specific Aim 1 will elucidate the role of calpains in axonal dysfunction and neurodegeneration after in vivo optic nerve stretch injury in rats. Adeno-associated viral vectors co-expressing EGFP will be used to over-express calpastatin or knock down u- and m-calpain expression in retinal ganglion cells, the source of optic nerve axons. Both functional and histopathological outcomes will be evaluated. Specific Aim 2 will use the same approach to discover the role of calpains in early microtubule disassembly and reassembly after in vivo optic nerve stretch in rats. Immunoelectron microscopy will be used to determine if calpains cause impaired microtubule reassembly and focal disruption of axonal transport (axonal swellings) after stretch injury. Specific Aim 3 will focus on the mechanism by which calpain-mediated proteolysis of microtubule components disrupts microtubule repolymerization. Stable calpain-derived proteolytic fragments of microtubule components will be identified and their function analyzed by generating corresponding truncated mutant proteins for in vitro and in vivo assays. Relevance: Every year in the US 2 million people suffer acute brain trauma. To reduce the morbidity and mortality of this disease, it is essential to understand the mechanisms underlying traumatic axonal injury.